A communications laser is frequently placed in a hermetic package that includes a photodiode optically coupled to the laser diode. The photodiode forms part of a feedback circuit, which samples a portion of the laser light to produce a photocurrent used to control the bias current of the laser in order to stabilize the light output power thereof. Typically these photodiodes are physically mounted very close to the laser, which results in a close thermal coupling between the laser and photodiode. In some instances, the laser and the photodiode may be at different temperatures, due to temperature changes outside the package in which they reside.
Cooled lasers used for fixed wavelength dense wavelength division multiplexing (DWDM) applications, which require control of wavelength to within +/−100 pm, normally include a thermistor within the laser housing to measure the internal temperature of the package.
The resistance of the thermistor, which varies as a nonlinear function of temperature, is used as a feedback mechanism to indirectly measure the laser temperature and to provide an indication of the laser wavelength. The resistance value of the thermistor is kept constant by use of a Peltier effect thermoelectric cooler (TEC) device inside the laser housing, upon which sits the thermistor, the monitor photodiode, and the laser. The feedback from the thermistor controls the current through the TEC to maintain a constant temperature resulting in a constant thermistor resistance value and a constant laser wavelength. The thermistor is typically considered be the weakest link in the wavelength control loop, i.e. a thermistor's drift with time is a main cause of wavelength drift in a D WDM laser source. A thermistor also requires space within the housing, adds two feed-throughs to the laser package, and increases cost and assembly complexity.
U.S. Pat. No. 5,266,792 in the names Crowne et al. entitled Temperature Compensated Optical Detector, issued November 1993, incorporated herein by reference, describes an optical wavelength detector wherein the use of a thermistor is obviated by using a photodiode as both a wavelength or intensity detector when reversed biased, and as a temperature sensor when forward biased. The circuit described by Crowne et al, includes means for ensuring that light from the lightsource being monitored is prevented from impinging upon the photoelectric feedback device when it is forward biased.
U.S. Pat. No. 5,024,535 in the name of Winston Jr. entitled Semiconductor Light Source Temperature Measurement also discloses the use of a photodiode in forward biased mode as a temperature sensor. A sensed voltage is converted into a temperature signal by the signal processor using a known temperature-voltage relationship of the sensing device.
Accordingly, it is well known that by simply using the output of the photodiode in a forward biased mode, an indication of temperature can be attained. Notwithstanding, more accurate results can be acquired by obviating the deleterious effect of allowing the light from the laser diode light source to couple into the photodetector when it is forward biased, sensing temperature. As mentioned heretofore, U.S. Pat. No. 5,266,792 provides means for interrupting the light source intermittently when temperature is being sensed. Although this solution may provide some advantages, during the measurement of temperature, the light source is switched off and the measurement is not a true measurement of the device with the laser on. Furthermore, data transmission must be halted during the time the light source is switched off; which is unacceptable in many instances. Furthermore, the circuitry required in this embodiment is more complex and less convenient to place in a small hermetic package. Moreover, the output wavelength of the laser diode will usually vary when the outside temperature surrounding the laser package changes. Therefore, if a forward biased photodiode is used as an indicator of temperature, an unwanted change in output wavelength of a laser diode thermally coupled to the detector may occur, as a result of a change in temperature outside the laser package, since the photodiode detector voltage-temperature curve is not a true indication of the temperature of the laser diode. More particularly, the operation of the all of the interconnected components and their related, but different, temperatures will affect the laser output signal wavelength. In the instance in which the forward biased photodiode presents a voltage indicating a corresponding temperature, using this voltage alone, as the only indicator that the output wavelength has or may have changed due to a temperature change, can result in the output wavelength of the laser drifting, even when it is desired to maintain the output at a constant wavelength.
It is an object of this invention, to provide a method and device that will maintain the output laser signal at a substantially constant wavelength by compensating for temperature changes outside the laser package, the voltage temperature relationship of the forward biased photodiode, any small difference in temperature of the detector and the laser diode, and the effects of the laser light impinging upon the forward biased detector while it is sensing temperature. By using outside temperature with the forward biased detector voltage to control a thermoelectric cooler, the output laser signal can be conveniently controlled. This also obviates having additional unwanted components such as a thermistor within the laser package.
In accordance with this invention, it has been found that comparing a voltage provided by the forward biased diode to a compensated voltage stored in memory related to the outside temperature about the package and related to the power of the laser diode, provides more accurate results and an output diode laser signal having a more stable wavelength.
The present invention provides a solution where the overall size of the device is reduced, cost is lessened and efficiency and longevity may be increased by elimination of components otherwise required by prior art devices.
It is an object of this invention to provide, within a laser package, a monitor of the laser sub-mount temperature that does not require the presence of a thermistor or other separate internal temperature measurement device. It is a further object of the invention to provide an output laser signal having a substantially constant wavelength while the light source remains on, in real-time, such that data transmission does not have to be halted. This invention can also be used for an un-cooled laser to provide an accurate monitor of the laser temperature in packages where a thermistor is not normally included. In the un-cooled laser application, accurate temperature feedback can be used to adjust drive or bias currents over temperature to maintain a consistent level of performance from the transmitter.